When Fruit Is Scarce, These Monkeys Hunt Animals. The Behavior May Offer Clues About the Origins of Humans' Meat-Eating Habits

The bearded capuchin monkeys of northeastern Brazil eat a mostly plant-based diet, chowing down on palm nuts and other fruits. But when plant foods are scarce, the primates consume a surprisingly diverse smorgasbord of small animals, including snakes, opossums, iguanas, bats, rodents and bird eggs and nestlings, researchers report in a study published on August 26 in the journal PLOS One. The findings reveal new details about the diet and behavior of these highly adaptable, long-lived monkeys.

But they could also shed light on the evolution of human diets, including our ancestors’ shift toward eating more meat. Hunting small prey seems to help capuchins cope with seasonal fluctuations in plant food availability—and “this may have also been the case for hominins,” study co-author Susana Carvalho, a primatologist at Gorongosa National Park in Mozambique, tells Science News’ Jake Buehler. When the researchers started studying wild capuchins in the privately owned area of Fazenda Boa Vista roughly two decades ago, they primarily wanted to investigate the monkeys’ tool use and other complex behaviors.

But while following two groups of the primates, scientists noticed that they were actively hunting and consuming small backbone-bearing creatures, or vertebrates—an intriguing behavior for a species that usually eats fruit. So, the researchers began making detailed notes whenever they saw the monkeys going after animals. They ended up with more than 5,000 hours of observations between May 2006 and December 2010, recording 446 instances of capuchins eating animals.

The team also looked at factors that may have affected the animals’ diets, including the age and sex of the individual monkeys that were chowing down on vertebrates, rainfall, temperature, humidity and the relative abundance of fruits and insects at the time. The overlaps revealed some clear patterns. When plant foods became scarcer due to seasonal fluctuations, the monkeys seemed to hunt more.

Lizards and rodents made up most of their prey, but the monkeys hunted a wide variety of animals. Scientists spotted males and females of all ages feasting on small vertebrates, but the behavior was more common among adult and juvenile males. And when their hunts were successful, the monkeys seemed to go for the brains and guts first, before eating the rest of the body.

Among primatologists, bearded capuchins are famous for using stone tools to obtain food. They’ve been observed placing palm nuts on a hard surface and smashing them with rocks to reveal the tasty kernels inside. Wild females have also been seen using sticks or blades of grass to pick their noses and eat any boogers they extract.